Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

geosciences

Article Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

Evgeniy Kozlov 1,* ID , Ekaterina Fomina 1, Mikhail Sidorov 1 and Vladimir Shilovskikh 2 ID

1 Geological Institute, Kola Science Centre, Russian Academy of Sciences, 14, Fersmana Street, 184209 Apatity, Russia; [email protected] (E.F.); [email protected] (M.S.) 2 Resource center for Geo-Environmental Research and Modeling (GEOMODEL), St. Petersburg State University, 1, Ulyanovskaya Street, 198504 Saint Petersburg, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-953-758-7632

Received: 6 July 2018; Accepted: 25 July 2018; Published: 28 July 2018

Abstract: This article is devoted to the geology of titanium-rich varieties of the Petyayan-Vara rare-earth dolomitic carbonatites in Vuoriyarvi, Northwest Russia. Analogues of these varieties are present in many carbonatite complexes. The aim of this study was to investigate the behavior of high field strength elements during the late stages of carbonatite formation. We conducted a multilateral study of titanium- and niobium-bearing minerals, including a petrographic study, Raman spectroscopy, microprobe determination of chemical composition, and electron backscatter diffraction. Three TiO2-polymorphs (anatase, brookite and rutile) and three pyrochlore group members (hydroxycalcio-, fluorcalcio-, and kenoplumbopyrochlore) were found to coexist in the studied rocks. The formation of these minerals occurred in several stages. First, Nb-poor Ti-oxides were formed in the fluid-permeable zones. The overprinting of this assemblage by residual fluids led to the generation of Nb-rich brookite (the main niobium concentrator in the Petyayan-Vara) and minerals of the pyrochlore group. This process also caused niobium enrichment with of early generations of Ti oxides. Our results indicate abrupt changes in the physicochemical parameters at the late hydro (carbo) thermal stage of the carbonatite formation and high migration capacity of Ti and Nb under these conditions. The metasomatism was accompanied by the separation of these elements.

Keywords: metasomatism; brookite; anatase; pyrochlore; niobium; carbonatites; Vuoriyarvi massif

1. Introduction Carbonatites are defined in the International Union of Geological Sciences (IUGS) system of classification as igneous rocks containing more than 50 modal percent carbonate [1]. Varieties of carbonatite are named based on the dominant carbonate mineral [2]. They have been chemically classified according to the percent by mass of CaO, MgO, and FeOT and MnO [1], which has an evolutionary aspect as well. Most of the known carbonatite complexes have a common scheme of carbonatite genesis, whereby early calciocarbonatites were replaced by magnesiocarbonatites, and ferrocarbonatites, as the most recent [3]. The latter are considerably affected by carbo(hydro)thermal-metasomatic processes as they develop [4]. In general, carbonatites account for more than 50% of global rare-earth elements (REE) resources, which include Y and La-Lu [5]. REE enrichment is “most commonly found only in the latest and most highly evolved parts of a carbonatite intrusion” [6]. The accumulation of REE in late carbonatites is assumed to be controlled by ﬂuid activity [7–16]. Such a surplus in articles reﬂects a keen interest in REE occurring in late carbonatites. More than 99% of the world’s niobium (Nb) is also

Geosciences 2018, 8, 281; doi:10.3390/geosciences8080281 www.mdpi.com/journal/geosciences Geosciences 2018, 8, 281 2 of 19 known to be associated with carbonatite complexes [17]. Still, only a limited number of works were dedicated to the behavior of high field strength elements (HFSE), which include Ti, Nb, Ta, Zr, and Hf, regardless of commercial value, at the final stages of the carbonatite genesis. The reason is that HFSE, unlike REE, are thought to be confined to the magmatic stage of the carbonatite genesis [18]. Moreover, a regular decrease in HFSE content occurs from early to late carbonatites [19]. However, this may not exclusively be the case. In most carbonatite deposits, the main niobium concentrators are minerals of the pyrochlore group and, less frequently, the perovskite, columbite, and euxenite groups [17]. The deposits are divided into primary (i.e., primarily magmatic) and secondary. The latter are richer and are supposed to have accumulated Nb minerals and other HFSE, due to their low mobility in the areas of laterite weathering above primary deposits. As such, late carbonatites have a specific HFSE-mineralization. Examples include the industrially valuable deposits of the Magnet Cove complex in the U.S. [20,21], carbonatites of the Salpeterkop complex in South Africa [22], and the Gross Brukkaros complex in Namibia [23]. Nb-rich polymorphs of TiO2 play a key role in all the listed deposits. These minerals (anatase, rutile, and brookite) are also found in many other complexes [24]. They are typomorphic for late carbonatites and reflect the process that completes the picture of late carbonatite petrogenesis. The evolution of the Morro dos Seis Lagos Nb (Ti, REE) deposit (Amazonas, Brazil) deserves particular attention. Here again, the main ore mineral is Nb-rich rutile. Reserves of this deposit are tremendous: they are up to 2.9 billion tons with 2.81 wt % Nb2O5 [25], which is much higher than in the largest known pyrochlore deposit. Hypergenesis certainly played a crucial role in the HFSE concentration process in the Morro dos Seis Lagos. However, laterites in this complex formed after ferro-(siderite)carbonatites that were originally rich in Ti and Nb [25]. In the Seis Lagos, like in many other complexes, Nb-bearing oxides are described as secondary phases replacing originally magmatic pyrochlores, perovskites, and other minerals. Still, this mechanism is not universal for carbonatites. Pseudomorphing of originally magmatic minerals is commonly associated with direct deposition of TiO2 polymorphs from fluids. In the latter case, more Nb often occurs in titanium phases, as described for the Gross Brukkaros [23]. Moreover, some examples have no pseudomorphs of rutile, brookite, or anatase after other phases. Only paramorphs of certain TiO2 modifications have been found, like in the Magnet Cove [26]. This calls into question the existing models of HFSE mineralization in carbonatites. Considering these challenges, decoding the processes of the late carbonatites petrogenesis is a major task. Determining these processes has both practical and fundamental value. The key step in the petrogenetic decoding is applying genetic mineralogy methods to Ti and Nb oxides [23,27–29]. The real challenge is dividing the late metasomatic changes and hypergenesis processes, which obscure earlier processes. The task becomes easier in the Vuoriyarvi. Weathering crusts that existed before the latest glaciations were eroded by glacial abrasion. Subsequently, almost no hypergenesis processes occurred due to the site’s location in the sub-Arctic region [30]. These “preserved” sites help to identify products of late hydrothermal alteration and those of hypergenetic processes [31]. Studying the evolution of the Ti-Nb mineralization in the Petyayan-Vara has supported a high migration capacity at the late hydrothermal stage of such immobile elements as Ti and Nb. We have also found evidence of fluid activity being accompanied by these elements’ fractionation.

2. Geological Setting of the Petyayan-Vara Rocks This research focused on the poorly studied carbonatites of the Petyayan-Vara area at the eastern ﬂank of the Vuoriyarvi alkaline-ultrabasic carbonatite complex, southwest of the Murmansk region, Northwest Russia. The Vuoriyarvi is one of the Devonian Kola Alkaline Province massifs [32]. Its concentrically zonal structure (Figure1) includes almost all rock varieties typical for such massifs [33]. Geosciences 2018, 8, 281 3 of 19 Geosciences 2018, 8, x FOR PEER REVIEW 3 of 19

Figure 1. The geological setting and structure of the Vuoriyarvi alkaline-ultrabasic carbonatite Figure 1. The geological setting and structure of the Vuoriyarvi alkaline-ultrabasic carbonatite complex complex after [33], simplified and added. Blue circles indicate positions of the Tukhta-Vara, Neske- after [33], simplified and added. Blue circles indicate positions of the Tukhta-Vara, Neske-Vara and Vara and Petyayan-Vara carbonatite fields and the Kolvikskiy agpaitic nepheline-syenite massif- Petyayan-Vara carbonatite fields and the Kolvikskiy agpaitic nepheline-syenite massif-satellite. satellite. Thus, at the current erosion section, the massif core is composed of olivitites (the first phase of Thus, at the current erosion section, the massif core is composed of olivitites (the first phase of intrusion) surrounded by pyroxenites (the intermediate ring, second phase) and foidolites (the outer intrusion) surrounded by pyroxenites (the intermediate ring, second phase) and foidolites (the outer ring, third phase). The next (fourth) stage of the magma intrusion produced nepheline syenites and ring, third phase). The next (fourth) stage of the magma intrusion produced nepheline syenites and related rocks concentrated in the Kolvikskiy massif-satellite mostly. The youngest formations of the related rocks concentrated in the Kolvikskiy massif-satellite mostly. The youngest formations of the Vuoriyarvi are stockwork-like bodies of various carbonatites (fifth phase) cutting other rocks and Vuoriyarvi are stockwork-like bodies of various carbonatites (fifth phase) cutting other rocks and forming several separated fields. Thus, near the Tukhta- and Neske-Vara tops, mostly calcite and, to a forming several separated fields. Thus, near the Tukhta- and Neske-Vara tops, mostly calcite and, to lesser extent, dolomite carbonatites and phoscorites are found. The Tukhta-Vara carbonatites include a lesser extent, dolomite carbonatites and phoscorites are found. The Tukhta-Vara carbonatites a deposit of apatite-magnetite-forsterite and rare metal (baddeleyite) ores. This deposit is close to include a deposit of apatite-magnetite-forsterite and rare metal (baddeleyite) ores. This deposit is the well-known Kovdor deposit (Kola Peninsula, Russia). On the Neske-Vara upland, along with close to the well-known Kovdor deposit (Kola Peninsula, Russia). On the Neske-Vara upland, along apatite-magnetite ores, a deposit of niobium and tantalum exists, including more than 110 tons of with apatite-magnetite ores, a deposit of niobium and tantalum exists, including more than 110 tons Ta2O5 reserves and 4750 tons of Nb2O5. Their major concentrators are pyrochlore, hatchettolite, of Ta2O5 reserves and 4750 tons of Nb2O5. Their major concentrators are pyrochlore, hatchettolite, and and zirkelite [33]. Thus, the Vuoriyarvi massif is a typical alkaline-ultrabasic carbonatite formation zirkelite [33]. Thus, the Vuoriyarvi massif is a typical alkaline-ultrabasic carbonatite formation both both structurally and compositionally. structurally and compositionally. Most carbonatite fields contain minor amounts of late varieties. The exceptions are the Most carbonatite fields contain minor amounts of late varieties. The exceptions are the Petyayan- Petyayan-Vara carbonatites that include pyroxenite-cutting veins and lenses. They are as long as several Vara carbonatites that include pyroxenite-cutting veins and lenses. They are as long as several hundred meters and tens of meters thick. The Petyayan-Vara carbonatites are spatially associated with hundred meters and tens of meters thick. The Petyayan-Vara carbonatites are spatially associated phlogopite glimmerites metasomatically formed after pyroxenites. The orientation and morphology of with phlogopite glimmerites metasomatically formed after pyroxenites. The orientation and the above carbonatite bodies are close to those of calcite carbonatites (soevites) from the Petyayan-Vara morphology of the above carbonatite bodies are close to those of calcite carbonatites (soevites) from surroundings. However, the carbonatite bodies differ from these rocks with their wide range of the Petyayan-Vara surroundings. However, the carbonatite bodies differ from these rocks with their mineralogical parageneses and a scope of mineralization, making them unique both for the Vuoriyarvi wide range of mineralogical parageneses and a scope of mineralization, making them unique both massif and for the Kola Alkaline Province in general. Magnesiocarbonatites are widespread within the for the Vuoriyarvi massif and for the Kola Alkaline Province in general. Magnesiocarbonatites are Petyayan-Vara field (Figure2), but unaltered primary magmatic dolomite carbonatites are rare. widespread within the Petyayan-Vara field (Figure 2), but unaltered primary magmatic dolomite carbonatites are rare.

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Figure 2. Composition points of the “titaniferous” carbonatites (blue triangles) and silicocarbonatites Figure 2.(redComposition squares), as well points as all of other the “titaniferous” carbonate rocks carbonatites of the Petyayan-Vara (blue triangles) area (green and circles) silicocarbonatites on the (red squares),classification as diagrams well asfrom all (a) Woolley other carbonateand Kempe [2] rocks and (b of) Gittins the and Petyayan-Vara Harmer [34]. Symbols: area (green circles)Cc—calcicocarbonatites, on the classiﬁcation Mc—magnesiocarbonatites, diagrams from (a) Woolley Fc—ferrocarbonatites, and Kempe and [2] Fcc—ferruginous and (b) Gittins and Harmercalciocarbonatites. [34]. Symbols: Cc—calcicocarbonatites, Mc—magnesiocarbonatites, Fc—ferrocarbonatites, and Fcc—ferruginous calciocarbonatites. Specific varieties of carbonatites are more common in the investigated carbonatite bodies. These are: Speciﬁc varieties of carbonatites are more common in the investigated carbonatite bodies. (1) Fine-grained dolomitic carbonatites and silicocarbonatites with microcline and/or phlogopite These are: (hereinafter called titaniferous carbonatites), locally enriched with apatite (carbonatites with K- (1) Fine-grainedAl-Si-Ti-Fe dolomitic ± P specialization); carbonatites and silicocarbonatites with microcline and/or phlogopite

(hereinafter(2) Medium-grained called titaniferous dolomitic carbonatites), carbonatites with locally barite, enriched strontianite, with apatite and (carbonatites ancylite-(Се) with (Ce,La)Sr(CO3)2(OH)×H2O ± bastnaesite-(Ce) (Ce,La)(CO3)F (carbonatites with Ba-REE-Sr K-Al-Si-Ti-Fe ± P specialization); specialization); and (2) Medium-grained(3) Breccias of dolomitic dolomitic carbonatites carbonatites with cement with consistingbarite, strontianite, of fine-grained and quartz ancylite-(Ce) and (Ce,La)hydroxylbastnaesite-(Ce)Sr(CO3)2(OH)×H2O (±Ce,bastnaesite-(Ce)La)(CO3)(OH) (carbonatites (Ce,La with)(CO REE-Si3)F (carbonatites specialization). with Ba-REE-Sr specialization);In the structure and of carbonatite bodies, these varieties are closely intertwined with each other, as (3) Brecciaswell as with of dolomiticprimary dolomitic carbonatites carbonatites with and cementglimmerites. consisting The most ofwidespread fine-grained are medium- quartz and hydroxylbastnaesite-(Ce)grained dolomitic carbonatites (Ce ,withLa)(CO baryte,3)(OH strontianite,) (carbonatites and ancylite-(Ce). with REE-Si All specialization).these minerals either corrode the primary magmatic dolomitic carbonatites or form cement in their breccias. In the latter In thecase, structure the texture ofof carbonatitethe rocks is similar bodies, to that these of breccias varieties of dolomitic are closely carbonatites intertwined with fine-grained with each other, as wellquartz-hydroxylbastnaesitic-(Ce) as with primary dolomitic cement. carbonatites Thus, most and of the glimmerites. Petyayan-Vara Thecarbonatites most are widespread typical are medium-grainedBa-Sr-REE-carbonatites dolomitic carbonatites with contents with of BaO baryte, up to strontianite,17 wt%, SrO up and to ancylite-(Ce).9 wt%, and REE All2O3 up these to 14 minerals wt%. Just like their protolith (magnesiocarbonatites), they are almost completely depleted in HFSE. either corrodeTitaniferous the primary carbonatites magmatic are also dolomitic common carbonatites and spatially orclose form to the cement marginal in theirparts breccias.of veins. In the latter case,They thestand texture out both of macroscopically the rocks is similarowing to totheir that specific of breccias reddish-brown of dolomitic color and carbonatites dense fine- with fine-grainedgrained quartz-hydroxylbastnaesitic-(Ce) structure (Figure 3a) and by their mineralogical-geochemical cement. Thus, most ofcharacteristics. the Petyayan-Vara It is not clear carbonatites at are typicalpresent Ba-Sr-REE-carbonatites whether magnesiocarbonatites with contents were their of BaOprotolith up toor if 17 they wt %,formed SrO (auto)metasomatically up to 9 wt %, and REE 2O3 up to 14after wt %.carbonatites Just like of their an additional protolith intrusion (magnesiocarbonatites), phase, or after host aluminosilicate they are almost rocks. completely The key role depletedof metasomatism in the evolution of titaniferous carbonatites is undoubted, as indicated by a widely in HFSE. distributed streaky mineralization, corrosion structures, amoeboid xenomorphic forms, and their Titaniferousloose spongy carbonatites texture (Figure are 3b). also This common is also confirmed and spatially by the pseudomorphic close to the substitution marginal partsof some of veins. They standminerals out by bothothers, macroscopically e.g., phlogopite by potassium owing to feldspar their (Figure specific 3c). reddish-brown Metasomatic recrystallization color and dense fine-grainedthat made structure the rocks (Figure fine-grained3a) and by and, their as amineralogical-geochemical result, poorly permeable by fluids. characteristics. Later associations, It is not clear at presentincluding whether Ba-Sr-REE, magnesiocarbonatites occur in titaniferous were carbonatites their protolith only along or cracks if they in formed veinlets and (auto)metasomatically in the cement after carbonatitesof brecciated of areas. an additional intrusion phase, or after host aluminosilicate rocks. The key role of metasomatism in the evolution of titaniferous carbonatites is undoubted, as indicated by a widely distributed streaky mineralization, corrosion structures, amoeboid xenomorphic forms, and their loose spongy texture (Figure3b). This is also confirmed by the pseudomorphic substitution of some minerals by others, e.g., phlogopite by potassium feldspar (Figure3c). Metasomatic recrystallization that made the rocks fine-grained and, as a result, poorly permeable by fluids. Later associations, Geosciences 2018, 8, 281 5 of 19 including Ba-Sr-REE, occur in titaniferous carbonatites only along cracks in veinlets and in the cement of brecciated areas. Geosciences 2018, 8, x FOR PEER REVIEW 5 of 19

Figure 3. (a) The relationship of Ba-Sr-rare earth elements (REE) (dark brown) carbonatites and Figure 3.titaniferous(a) The (reddish-brown) relationship ofcarbonatites, Ba-Sr-rare as well earth as ancylite-rich elements (REE) (red) areas; (dark (b) brown) groundmass carbonatites texture and titaniferousof the (reddish-brown) typical titaniferous carbonatites, carbonatite, amoeboid as well as xenomorphic ancylite-rich forms (red) of areas; minerals, (b )and groundmass their loose texture of the typicalspongy titaniferoustexture; (c) substitution carbonatite, of microcline amoeboid by xenomorphic phlogopite; and forms (d) TiO of2-carbonate minerals, (dolomite and their + loose calcite) veinlets in the dolomite carbonatite. Image (b) in backscattered electrons (BSE), (c,d) are in spongy texture; (c) substitution of microcline by phlogopite; and (d) TiO2-carbonate (dolomite + calcite) veinletstransmitted in the dolomite light [top—in carbonatite. plane-polarized Image (b light,) in backscattered bottom—in cross-polarized electrons (BSE), light]; ( cCal:,d) calcite, are in transmittedDol: dolomite (the numbers correspond to the generations, see explanation in the text), Mc: microcline, Qz: light [top—in plane-polarized light, bottom—in cross-polarized light]; Cal: calcite, Dol: dolomite quartz, Phl: phlogopite, Ti ox.: titanium oxides (anatase, brookite and/or rutile), and Fe ox.: oxides- (the numbershydroxides correspond of Fe. Scale to rulers the generations, are 200 μm in seelength. explanation in the text), Mc: microcline, Qz: quartz, Phl: phlogopite, Ti ox.: titanium oxides (anatase, brookite and/or rutile), and Fe ox.: oxides-hydroxides of Fe. ScaleTitanium rulers oxides are 200 areµ mrock-forming in length. minerals of titaniferous carbonatites. Geochemically, their presence reflects highly TiO2-enriched rocks (0.9–4.4 wt% with median [Me] of 2.3 wt% for 11 samples). Titanium varieties are also rich in SiO2 (8.1–35.1 [Me 21.8] wt%), Al2O3 (2.6–7.7 [Me 3.9] Titanium oxides are rock-forming minerals of titaniferous carbonatites. Geochemically, their presence wt%), and K2O (2.0–5.9 [Me 3.5] wt%), which is not typical of carbonatites. At the mineralogical level, reflectsthese highly geochemical TiO2-enriched characteristics rocks are (0.9–4.4 reflected wt by, % withalong medianwith ferruginous [Me] of dolomite 2.3 wt and % for calcite, 11 samples).a Titaniumsodium-free varieties microcline, are also richphlogopite in SiO and,2 (8.1–35.1 in minor [amounts,Me 21.8] albite, wt %), titanium Al2O 3aegirine,(2.6–7.7 and [Me quartz.3.9] wt %), Interestingly, sulfides (mainly pyrite) are only widespread in titaniferous carbonatites only, whereas and K2O (2.0–5.9 [Me 3.5] wt %), which is not typical of carbonatites. At the mineralogical level, these geochemicalin later metasomatites, characteristics almost all are sulphur reflected is sulphate by, alongand occurs with in ferruginousbarite. According dolomite to the current and calcite, IUGS classification [1], varieties with more than 20 wt% SiO2 are silicocarbonatites. In the studied a sodium-free microcline, phlogopite and, in minor amounts, albite, titanium aegirine, and quartz. area, silicocarbonatites only occur among titaniferous rocks, with microline containing the greatest Interestingly,amount sulfides of silica. Among (mainly titaniferous pyrite) are rocks, only some widespread samples are in highly titaniferous rich in Th-Y-apatite carbonatites (P2O only,5 up to whereas in later7.5 metasomatites, wt%), though the almost phosphorous all sulphur content is in sulphate most of these and occursrocks is not in barite.that high According (Me 0.5 wt%). to theThe current IUGS classificationcontents vary [1 since], varieties the apatitization with more is than imposed 20 wt on % the SiO Ti-alkaline2 are silicocarbonatites. alumosilicate association. In thestudied area, silicocarbonatitesCompared to the Petyayan-Vara only occur amongBa-Sr-REE-carbonatites, titaniferous rocks, Ti-rich with varieties microline have low containing concentrations the of greatest BaO (0.1–2.8 [Me 0.2] wt%), SrO (up to 0.9 [Me 0.1] wt%), and REE2O3 (0.2–1.4 [Me 0.3] wt%). The amount of silica. Among titaniferous rocks, some samples are highly rich in Th-Y-apatite (P2O5 up concentrations are close to those in dolomite carbonatites. The Petyayan-Vara titaniferous to 7.5 wtcarbonatites %), though show the the phosphorous highest concentrations content of in iron, most localized of these in the rocks ankerite is not component that high of dolomite (Me 0.5 wt %). The contents vary since the apatitization is imposed on the Ti-alkaline alumosilicate association.

Compared to the Petyayan-Vara Ba-Sr-REE-carbonatites, Ti-rich varieties have low concentrations of BaO (0.1–2.8 [Me 0.2] wt %), SrO (up to 0.9 [Me 0.1] wt %), and REE2O3 (0.2–1.4 [Me 0.3] wt %). Geosciences 2018, 8, 281 6 of 19

The concentrations are close to those in dolomite carbonatites. The Petyayan-Vara titaniferous carbonatites show the highest concentrations of iron, localized in the ankerite component of dolomite and in Fe oxides-hydroxides, providing the rocks with their specific coloring. As a result, in CMF classification charts (CaO − MgO − FeOT + MnO) [2,34], figurative points of titaniferous carbonatites shift toward the field of ferrocarbonatites (Figure2). At a distance of a few meters from titaniferous varieties, “ordinary” dolomite carbonatites there have fine TiO2-carbonate (dolomite and calcite) veinlets that formed along joints (Figure3d). The only feature that differentiates dolomite carbonatites with such veinlets from unaltered dolomite carbonatites is an increased TiO2 content (up to 1.1 wt %). The veinlets per se indicate the mobility of titanium in late stage fluids. In isolated instances, in dolomite carbonatites from the rimming of titaniferous varieties, jointing controls the pyrochlore mineralization without TiO2-associated enrichment. In this case, the concentration of Nb increased up to 1500 ppm with the average content of 400–800 ppm in titaniferous varieties and 10–100 ppm in other carbonatites of the Petyayan-Vara.

3. Research Material and Analytical Methods Eleven samples of K-feldspar- and/or phlogopite-bearing titaniferous carbonatites and silicocarbonatites were sampled from three carbonatite bodies of the Petyayan-Vara as a study material. Several samples of dolomite carbonatites from the rimming of titaniferous varieties were studied that contained the titanium and niobium veinlet mineralization. Titanium oxides (anatase, brookite, and rutile) are barely discernible both optically and in terms of their chemical composition. Therefore, we have studied them using methods based on structural characteristics of minerals: Raman spectroscopy (RS) and electron back scattered diffraction (EBSD). The former method is efficient due to differences in the Raman spectra of TiO2 polymorphs [35]. The Raman spectra were recorded on a HORIBA Jobin-Yvon LabRam HR800 laser Raman spectrophotometer (Horiba, Kyoto, Japan) on the Olympus confocal microscope (100× lens) in the range of 50–4000 cm−1 with a spectral resolution of 2 cm−1. The Raman scattering signal was excited by a green laser (514.5 nm) with the power of 50 mW (acquisition time 4–5 s, 10–15 acquisitions). The calibration was checked using a Raman line of silicon at 521 cm−1. Being highly productive, RS has been used for express identification of microprobe test points. Using the electron backscatter diffraction (EBSD) allowed for the visualization of the TiO2 polymorphs ratios and defining boundaries of individual grains in intergrowths, where grains are constrained by mineral sections with a single crystallographic orientation in space. For EBSD measurements, a Hitachi S-3400N scanning electron microscope equipped with Oxford HKLNordlys EBSD detector was used. Operating conditions were as follows: accelerating voltage 20 kV, beam current 1 nA, acquisition time 16 s, and step 2 µm for one EBSD during mapping. Oxford Instruments AZtecHKL analysis software was used to identify the crystal orientation from the Kikuchi pattern, using the crystal structures of rutile, anatase and brookite from the Inorganic Crystal Structure Database. In order to obtain the flattest surface possible for an EBSD analysis, a sample was polished with an ending stage of Ar-ion etching for 10 min (Oxford IonFab 300). The chemical heterogeneity of minerals was analyzed using BSE-images and maps of element distribution provided by scanning electron microscopes (SEM). The chemical composition of the minerals was detected using SEM Hitachi S-3400N with Oxford X-Max 20 energy dispersive X-ray spectrometer (EDS) (20 kV, 1 nA, 30 s per exposure) and a wavelength-dispersive spectrometer (WDS) Cameca MS-46 electron microprobe (22 kV, 30 nA, 50 s per exposure). EDS data were gathered with no standards and WDS data were calibrated using natural and synthetic standards. Two phases and lines were selected to calibrate the data: wollastonite (SiKα, CaKα), anatase (TiKα), hematite (FeKα), apatite (PKα), lorenzenite (NaKα), wadeite (KKα), zircon (ZrLα), thorite (ThMα), metallic Nb (NbLα), metallic Ta (TaLα), metallic U (UMα), SrSO4 (SrLα), BaSO4 (BaLα), PbMoO4 (PbLα), Y3Al5O12 (YLα), (La,Ce)S (LaLα), CeS (CeLα), LiNd(MoO4)2 (NdLα), SmFeO3 (SmLα). The detection limit for Si, Ti, Ca, Fe, K, and Cl was 0.02%; 0.05% for P, Na, Sr, Ba, Ta, Y, La, Ce, Nd, Pr, and Sm; and 0.1% for Zr, Nb, U, Geosciences 2018, 8, 281 7 of 19

Th, and Pb. The data were reduced using the original matrix correction technique [36]. The ﬂuorine content was detected using a LEO-1450 SEM equipped with an XFlash-5010 Bruker Nano GmbH EDS (20 kV, 0.5 nA, 200 s per exposure). Data reduction was performed using standard-free analysis based on the P/B-ZAF method of the QUANTAX system. The comparison of the results obtained from WDS (with standards) and EDS (without standards) on pyrochlores and titanium oxides showed that normalized results were close for both minerals, though the EDS analyses indicated underweight. The value of Ti + Nb + FeT is given as 1.00 atom per formulae unit (a.p.f.u.), when crystallochemical formulas of titanium oxides are calculated [23]. Fe2+ and Fe3+ ratios were not evaluated for the reason provided in Section 4.1. Crystallochemical formulas of the pyrochlore group minerals share a common 2+ 2+ 2+ 3+ view: A2−m B2 X6−wY1−n, where A = Na, Ca, Sr, Ba, Fe , Pb , Sn , Sb , Y, Ce (and other REE), U, 5+ 5+ 4+ 3+ Th, or H2O; B = Ta, Nb, Ti, Sb , W, V , Sn , Zr, Hf, Fe and Si; X typically is O, but can include – subordinate OH and F; Y typically is OH , F, O, ,H2O[37]. The formulas were calculated based on the notion that the B-site has no vacancies; therefore, the Nb + Ta + Ti + Zr + Si + FeT sum is 2.00 a.p.f.u. O and OH were calculated considering the balance of charge and stoichiometry. Since FeT content increased by 0.05 a.p.f.u. in only in a few probes, like in [28], we have also decided not to divide Fe2+ and Fe3+.

4. Results

4.1. Titanium Minerals In titaniferous carbonatites, glomeroblast clusters of titanium oxides grains are tightly associated with xenomorphic individuals of K-feldspar and F-rich phlogopite laths, if the latter are present. Raman spectroscopy indicated Ti oxides occur as three polymorph modifications: anatase, rutile, and brookite. Brookite predominates in titaniferous carbonatites, anatase is minor, and rutile produces paramorphs after both minerals. Brookite is dark brown to yellowish brown in transmitted light, anatase is indigo, and rutile is brown or blood red. However, these minerals are commonly grouped in morphologically different muddy brown aggregates that hamper optic diagnostics. Five morphotypes of titanium oxide isolations were detected in titaniferous carbonatites: segregations of elongated crystals (I type), xenomorphic rims of carbonatite grains overgrowths (II type), amoeboid isolations (III type), specific “cementing” intergrowths with sulfides (IV type), and small idiomorphic individuals of brookite (V type) (Figure4a–e, respectively). I–IV type isolations occur mostly in a rock tissue or less frequently within K-feldspatized carbonate veinlets. These transformed veinlets are often visible only due to their shady structure accentuated by relic areas of late dolomite adjacent to selvages. This dolomite differs from the muddy spongy one of the main rock tissue with its dense structure, idiomorphic appearance, and fine oscillation zoning. The V type idiomorphic brookite was solely observed in by-selvage areas of well-preserved cutting calcite veinlets associated with a late generation of dolomite. Note, I type segregations are also composed preliminary of brookite (Figure4f), although elongated crystals are typical for anatase and rutile. Amoeboid isolations of TiO2 (III type) are the most widespread. They provide a full view of the mineral genesis as they include three polymorphs in their structure. Anatase grains occur as fragments, with the biggest ones in the brookite mass and the smaller ones occurring outside as constellations (Figure5a). Brookite constitutes more than 80 vol% of isolations, which corresponds with its share in the balance of TiO2 for titaniferous carbonatites. Brookite is found to have formed in at least two stages. Earlier generation brookite, Brk-1 is chemically homogenous (Figure5b) upon which radial paramorphs of rutile developed. Rutile is likely to have a cell structure (Figure5c) due to different densities of brookite (4.08–4.18 g sm−3) and rutile (4.23–5.50 g sm−3)[38] which as a result, decrease in its volume. Rutile differs from other titanium oxides with an increased content of admixtures: Ca, Si, as well as P and Na (according to a number of analyses). We presumed it to result from calcite, aegirine, and apatite filling extra volume that occurred in the paramorph substitution. The above Geosciences 2018, 8, 281 8 of 19 minerals compose the paragenetic association of the apatitization. The latter, as stated, is imposed on the rock-forming association of titaniferous carbonatites. Geosciences 2018, 8, x FOR PEER REVIEW 8 of 19

Geosciences 2018, 8, x FOR PEER REVIEW 8 of 19

Figure 4. Morphotypes of titanium oxide isolations: (a) I type—segregations of elongated crystals, (b) Figure 4. Morphotypes of titanium oxide isolations: (a) I type—segregations of elongated crystals, II type—xenomorphic rims of carbonatite grains overgrowths, (c) III type—amoeboid isolations, (b) II type—xenomorphic rims of carbonatite grains overgrowths, (c) III type—amoeboid isolations, “bright”Figure 4. mineral Morphotypes is monazite of titanium (Mnz), oxide (d) IV isolations: type—specific (a) I type—segregations “cementing” intergrowths of elongated with crystals,pyrite (Py), (b) “bright”andII mineral type—xenomorphic (e) V type—small is monazite rimsidiomorphic (Mnz), of carbonatite (d )individuals IV type—speciﬁc grains of overgrowths,brookite. “cementing” (f) Electron (c) III type—amoeboid intergrowthsback scattered diffraction with isolations, pyrite (Py), and (e) V(EBSD)“bright” type—small phases mineral map idiomorphic is monazite of the I (Mnz),type individuals of (d titanium) IV type—specific of oxidebrookite. isolations, “cementing” (f) Electron hereinafter intergrowths back on scattered EBSD with phases diffractionpyrite maps (Py), (EBSD) phases map(Figuresand (e of) Vthe 5a,b type—small I and type 6b): of green—dolomitetitaniumidiomorphic oxide individuals (Dol), isolations, yellow—brookite of brookite. hereinafter (f) Electron (Brk), on EBSD red—anatase back phases scattered (Ant), maps diffraction blue— (Figure 5a,b rutile (Rt). Images (4a–e) are in backscattered electrons (BSE). Scale rulers are 200 μm long. and Figure(EBSD)6b): phases green—dolomite map of the I (Dol),type of yellow—brookite titanium oxide isolations, (Brk), hereinafter red—anatase on EBSD (Ant), phases blue—rutile maps (Rt). Images(Figures (4a–e) are 5a,b in and backscattered 6b): green—dolomite electrons (Dol), (BSE). yellow—brookite Scale rulers are (Brk), 200 red—anataseµm long. (Ant), blue— rutile (Rt). Images (4a–e) are in backscattered electrons (BSE). Scale rulers are 200 μm long.

Figure 5. (a,b) Anatomy of amoeboid isolations of TiO2 (III type), contrasting BSE-images. On the inserts are phase distribution maps within the same sections, obtained by the EBSD; (c) substitution of brookite by rutile; and (d) Euler angle coloring orientation map for the (b) section, colors reflect the Figure 5. (a,b) Anatomy of amoeboid isolations of TiO2 (III type), contrasting BSE-images. On the Figure 5.proximityinserts(a,b are) Anatomy of phase the orientation distribution of amoeboid of maps crystals. within isolations Scale the rulers same of are TiOsections, 2002 (IIIμm obtained type),long. contrastingby the EBSD; BSE-images.(c) substitution On the inserts areof brookite phase distributionby rutile; and ( mapsd) Euler within angle coloring the same orientation sections, map obtained for the ( byb) section, the EBSD; colors (c reflect) substitution the of brookite proximity by rutile; of the and orientation (d) Euler of angle crystals. coloring Scale rulers orientation are 200 μm map long. for the (b) section, colors reﬂect the proximity of the orientation of crystals. Scale rulers are 200 µm long.

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LaterLater generation generation brookite, brookite, Brk-2 Brk-2 overgrowths overgrowths both both Brk-1, Brk-1, and and the rutile developed on it. Its Its Nb contentcontent also also varies, providing the mineral with a mosaic appearance appearance in BSE-images. BSE-images. However, However, accordingaccording to thethe EBSDEBSD resultsresults (Figure (Figure5 d),5d), despite despite the the block block structure structure of amoeboidof amoeboid isolations, isolations, they they are arecomposed composed of individual of individual monocrystals, monocrystals, where where brookites brookites of both of both generations generations are jointed.are jointed. The content of Nb O varied from 0.7 to 5.2 wt % in the studied grains of titanium oxides. The content of Nb2O2 5 varied5 from 0.7 to 5.2 wt% in the studied grains of titanium oxides. Given theGiven maximum the maximum distribution, distribution, brookite brookite is the is themain main concentrator concentrator of of niobium niobium in in the the Petyayan-Vara Petyayan-Vara carbonatites.carbonatites. The distribution ofof niobiumniobium inin thethe studied studied sampling sampling was was bimodal bimodal with with clear clear maximums maximumsof of2.2 2.2 wt % wt% and and 4.3 wt 4.3 % wt% (Figure (Figure6a). As 6a). showed As showed Werner and Werner Cook and [ 23], Cook in rocks [23], similar in rocks to the similar Petyayan-Vara to the Petyayan-Varacarbonatites, titanium carbonatites, oxides titanium included oxides Nb and included Fe under Nb and two Fe related under schemes two related of isomorphism: schemes of 4+ 5+ 3+ 4+ 5+ 2+ isomorphism:2Ti Nb 2Ti+ Fe4+ ⇄ Nband5+ + 3TiFe3+ and 3Ti2Nb4+ ⇄ 2Nb+ Fe5+ +. Fe In2+. chartIn chart at coordinatesat coordinates Nb Nb a.p.f.u. a.p.f.u.− − Fe a.p.f.u. a.p.f.u. (Figure(Figure 66b)b)the the field field indicating indicating combined combined impacts impacts of of the the above above schemes schemes is bounded is bounded bylines by lines with with 0.5 and 0.5 and1.0 inclines. 1.0 inclines. Less Less than than one-third one-third of figurative of figurative points points were were found found in this in this field; field; the restthe rest were were divided divided into intoequal equal groups groups of those of those with“excessive with “excessive Fe” (higher Fe” (higher than the than line withthe line incline with 1) incline and those 1) and with those “excessive with “excessiveNb” (lower Nb” than (lower the line than with the incline line with 0.5). incline The appearance 0.5). The appearance of excessive of iron excessive is a common iron is featurea common that featurecan be explainedthat can by be mechanically explained by included mechanically Fe oxides-hydroxides. included Fe oxides-hydroxides. Examples include Examples brookite and include rutile brookitefrom the and Nb (Ti,rutile REE) from deposit the Nb Morro (Ti, REE) dos deposit Seis Lagos Morro ([25 ];dos see Seis their Lagos Figure ([25]; 12). see The their unusual Figure pattern 12). The of unusualthe excessive pattern Nb of entry the excessive that is not Nb compensated entry that is not by Fecompensated requires further by Fe researchrequires further [39]. Thus, research attempts [39]. 2+ 3+ 2+ 2+ 3+ Thus,at balancing attempts Fe at and balancing Fe according Fe2+ and to Fe the3+ according scheme Fe to /(Fethe scheme+ Fe Fe) ≈2+/(Fe1 −2+ (Nb/Fe) + Fe3+) ≈ suggested 1 − (Nb/Fe) by suggestedWerner and by Cook Werner [23 ]and have Cook failed [23] and have need failed further and refinement. need further refinement.

Figure 6. (a) Histogram of Nb2O5 distribution and (b) binary diagram Ti vs. Nb + Fe3+ for the titanium Figure 6. (a) Histogram of Nb O distribution and (b) binary diagram Ti vs. Nb + Fe3+ for the titanium oxides of the Petyayan-Vara titaniferous2 5 carbonatites. oxides of the Petyayan-Vara titaniferous carbonatites. Titaniferous carbonatites demonstrated a steady trend in the Nb-enrichment of the later generationTitaniferous of brookite. carbonatites In the demonstratedsurrounding primary a steady dolomite trend in the carbonatites, Nb-enrichment the TiO of the2-dolomite-calcite later generation veinletsof brookite. contains In the titanium surrounding oxides with primary a different dolomite distribution carbonatites, of Nb. the Unlike TiO2-dolomite-calcite titaniferous carbonatites, veinlets thesecontains phases titanium were easily oxides optically with a detected different here distribution (Figure 7a). of Nb.In such Unlike veinlets, titaniferous anatase and carbonatites, brookite occuredthese phases in equal were shares easily and optically are grouped detected in glomeroblast here (Figure isolations,7a). In suchwhereas veinlets, rutile anatasewas scarce and (Figure brookite 7b). Morphologically,occured in equal these shares glomers and are are groupedclose to the in I glomeroblasttype isolations isolations, of TiO2 from whereas titaniferous rutile carbonatites was scarce (cf.(Figure Figures7b). Morphologically,4a and 7c). Anatase these produces glomers well-facetted are close to the fusiform I type isolations crystals, ofwhereas TiO 2 from brookite titaniferous either includescarbonatites it with (cf. Figuresa poikilitic4a and texture,7c). Anatase or co-exists produces as idiomorphic well-facetted isometric fusiform crystals,grains. BSE-images whereas brookite of the discussedeither includes glomers it with provided a poikilitic a clear texture, picture or co-existsof rims impregnating as idiomorphic the isometric edges of grains. both anatase BSE-images and brookiteof the discussed (Figure 7d). glomers When provided untouched a clear by these picture rims, of anatase rims impregnating and brookite the are edges low in of niobium, both anatase and earlierand brookite anatase (Figure was poorer7d). When than untouchedbrookite (on by average, these rims, 0.2 wt% anatase and and 0.5 brookitewt% Nb2 areO5 lowrespectively). in niobium, In rimmingsand earlier of anatase both minerals, was poorer the than Nb content brookite dramatically (on average, increased 0.2 wt % and up 0.5to 2.5 wt wt% % Nb Nb2O52Orespectively).5 in anatase and 5.0 wt% in brookite.

Geosciences 2018, 8, 281 10 of 19

In rimmings of both minerals, the Nb content dramatically increased up to 2.5 wt % Nb2O5 in anatase and 5.0Geosciences wt % 2018 in brookite., 8, x FOR PEER REVIEW 10 of 19

Figure 7. TiO2-carbonate veinlets in the dolomite carbonatite: (a) titanium oxides in transmitted light Figure(top—in 7. TiO 2 plane-polarized-carbonate veinlets light, in bottom—in the dolomite cross-polarized carbonatite: ( light),a) titanium (b) EBSD oxides phases in transmitted map, (c) light (top—inmorphology plane-polarized of titanium light, oxide bottom—in isolations cross-polarizedon the BSE image, light), and (d (b) )an EBSD example phases of the map, distribution (c) morphology of of titaniumNb2O5 in oxide the impregnating isolations onrims the both BSE in anatase image, and and in ( dbrookite.) an example In square of brackets: the distribution contents of of Nb Nb2O25O 5 in the impregnating(wt%). Scale rulers rims are both 100 in μm anatase long. and in brookite. In square brackets: contents of Nb2O5 (wt %). Scale rulers are 100 µm long. 4.2. Niobium Minerals 4.2. NiobiumNiobium Minerals minerals in the Petyayan-Vara carbonatites are represented solely by members of the pyrochlore group. In titaniferous varieties individual idiomorphic grains of well-preserved honey- Niobium minerals in the Petyayan-Vara carbonatites are represented solely by members of yellow pyrochlore were detected in the by-selvedge areas of cutting calcite veinlets associated with a the pyrochlorelater generation group. of dolomite. In titaniferous Namely, pyrochlore varieties individualmaintains the idiomorphic same structural grains position, of well-preserved as V-type honey-yellowidiomorphic pyrochlore brookite, werewhich detected is commonly in the associated by-selvedge with areas the latter. of cutting Most calcite grains veinlets are zonal: associated (1) withTh-variety a later generation with excessive of dolomite. Si and no Namely, F that compose pyrochlore the submicron maintains embryos the same in a crystal structural core, position, (2) the as V-typemain idiomorphic volume is brookite, occupied whichby Ca-F-pyrochlore is commonly substituted associated along with rimsthe latter. and cracks, Most grains and (3) are by zonal: Pb- (1) Th-varietypyrochlore with without excessive fluorine Si and (Figure no F that 8a,b). compose Both embryos the submicron and rims embryos were not in detected a crystal in core,every (2) single the main volumecrystal. is occupied Rims obviously by Ca-F-pyrochlore tended toward substitutedthe latest minerals, along rimssuch as and monazite. cracks, According and (3) by to Pb-pyrochlore the IMA withoutnomenclature, fluorine (Figure cores,8 intermediatea,b). Both embryos areas, and and rims rims of were pyrochlore, not detected the compositions in every single of which crystal. are Rims provided in Table 1, refer to U-Th-rich hydroxycalciopyrochlore (Ca,Sr,Th,U,□)2(Nb,Ti)2O6(OH), obviously tended toward the latest minerals, such as monazite. According to the IMA nomenclature, fluorcalciopyrochlore (Ca,Na)2(Nb,Ti)2O6(F,OH), and kenoplumbopyrochlore (Pb,□)Nb2O6(□,O), cores, intermediate areas, and rims of pyrochlore, the compositions of which are provided in Table1, respectively [37]. Studying U-Th- and Pb-varieties (in cores and rimes, respectively) with the WDS refermethod to U-Th-rich has failed hydroxycalciopyrochlore because of their small sizes. (Ca Their,Sr,Th compositions,U,)2(Nb,Ti were)2O6 only(OH ),evaluated fluorcalciopyrochlore with EDS, (Ca,Nawhich)2(Nb was,Ti) still2O6 rather(F,OH accurate.), and kenoplumbopyrochlore First, the measurements did (Pb not, include)Nb2O6 neighboring(,O), respectively Ca-F-pyrochlore, [37]. Studying as U-Th-testified and Pb-varieties by the absence (in of cores F in andboth rimes,analyses respectively) and the absence with of the Ca WDSin that methodof Pb-pyrochlore. has failed Second, because of theirthe small measured sizes. Their compositions compositions were close were to only pyrochlores evaluated from with other EDS, similar which complexes, was still rather as will accurate. be First,highlighted the measurements in Section did 5. We not could include not explain neighboring the excessive Ca-F-pyrochlore, silica in the composition as testified of by the the U-Th-rich absence of F in bothvariety. analyses As shown and theby the absence study of the Ca silicified in that ofpyrochlore Pb-pyrochlore. from the Second, Narssârssuk, the measured Julianehaab compositions district, wereGreenland, close to pyrochlores conducted by from Bonazzi other et similar al. [39], complexes, “50–70% of asthe will total be silicon highlighted detected in is Sectionincorporated5. We in could

Geosciences 2018, 8, 281 11 of 19 not explain the excessive silica in the composition of the U-Th-rich variety. As shown by the study of Geosciences 2018, 8, x FOR PEER REVIEW 11 of 19 the siliciﬁed pyrochlore from the Narssârssuk, Julianehaab district, Greenland, conducted by Bonazzi etthe al. radiation-damaged [39], “50–70% of the portions total silicon of pyrochlore”. detected is In incorporated our case, we in consider the radiation-damaged thorium responsible portions for the of pyrochlore”.radiation-damaged In our case,portions. we consider thorium responsible for the radiation-damaged portions.

Figure 8. (a) U-Th-rich hydroxycalciopyrochlore (U-Th-OH Pcl) submicron embryos, surrounded by Figure 8. (a) U-Th-rich hydroxycalciopyrochlore (U-Th-OH Pcl) submicron embryos, surrounded by fluorcalciopyrochlore, (Ca-F Pcl); (b) euhedral crystals of fluorcalciopyrochlore in association with fluorcalciopyrochlore, (Ca-F Pcl); (b) euhedral crystals of fluorcalciopyrochlore in association with small small idiomorphic individuals of brookite (V type of TiO2), rims of kenoplumbopyrochlore (Pb Pcl) idiomorphic individuals of brookite (V type of TiO2), rims of kenoplumbopyrochlore (Pb Pcl) around around fluorcalciopyrochlore; (c) chain-like pyrochlore mineralization associated with sulphides, Fe- fluorcalciopyrochlore; (c) chain-like pyrochlore mineralization associated with sulphides, Fe-oxides, oxides, and phlogopite in pyrochlore-rich areas near titaniferous varieties in dolomite carbonatites; and phlogopite in pyrochlore-rich areas near titaniferous varieties in dolomite carbonatites; and (d) and (d) resorption of euhedral pyrochlore on contact with phlogopite and substitution of resorption of euhedral pyrochlore on contact with phlogopite and substitution of fluorcalciopyrochlore fluorcalciopyrochlore (near the contact) by kenoplumbopyrochlore, associated with monazite.. (near the contact) by kenoplumbopyrochlore, associated with monazite.

As mentioned in Section 2, pyrochlore-rich areas were found near the titaniferous varieties in dolomiteAs mentioned carbonatites. in Section Here,2 , pyrochlore pyrochlore-rich was areas compositionally were found similarnear the with titaniferous fluorcalcio- varieties and inkenoplumbopyrochlore dolomite carbonatites. from Here, titaniferous pyrochlore rocks was (Table compositionally 1). Judging by similar the structural with fluorcalcio- features, these and kenoplumbopyrochloreareas dinamometamorphically from titaniferous changed rocks before (Table the1 ). pyrochlore Judging by mineralization. the structural features, As a result, these areas they dinamometamorphicallyformed a schistose fine-grained changed before cataclastic the pyrochlore structure enriched mineralization. by magnetite. As a result, The they pyrochlore formed amineralization schistose fine-grained was distributed cataclastic in structure a chain-like enriched manner by magnetite. and constrained The pyrochlore by cracks mineralization cutting the wasschistosity distributed (Figure in a 8c). chain-like Sulfides manner (pyrite) and and constrained Fe oxides-hydroxides by cracks cutting developed the schistosity along these (Figure cracks.8c). SulfidesXenomorphic (pyrite) laths and of Fephlogopite, oxides-hydroxides which seemed developed to have alongoccurred these after cracks. pyrochlore, Xenomorphic spatially laths tended of phlogopite,toward small which areas seemed of the pyrochlore to have occurred mineralization. after pyrochlore, At the contact spatially with tended mica pyrochlore toward small was areas clearly of theresorbed pyrochlore and always mineralization. substituted At theby Pb-pyrochlore contact with mica at edges. pyrochlore Pb-pyrochlore was clearly (Figure resorbed 8d) and developed always substitutedinwards along by Pb-pyrochlore cracks and, in at segregations edges. Pb-pyrochlore of pyrochlore (Figure grains,8d) developed crystal boundaries. inwards along In dolomite cracks and,carbonatites in segregations with chain-like of pyrochlore pyrochlore grains, mineralization, crystal boundaries. no anatase, In dolomite brookite, carbonatites or rutile withwas chain-likeobserved. pyrochloreThese rocks mineralization, contained the Sr-Ba-REE no anatase, mineralization brookite, or rutilerepresented was observed. by ancylite, These barite, rocks and contained bastnaesite. the Sr-Ba-REEThe minerals mineralization are late in terms represented of pyrochlore by ancylite, and barite, phlogopite. and bastnaesite. Notably, late The monazite minerals areoften late occurs in terms in ofthe pyrochlore studied dolomite and phlogopite. carbonatites Notably, with late themonazite chain-like often pyrochloreoccurs in the mineralization, studied dolomite ascarbonatites well as in titaniferous varieties, in association with Pb-pyrochlore rims.

Geosciences 2018, 8, 281 12 of 19 with the chain-like pyrochlore mineralization, as well as in titaniferous varieties, in association with Pb-pyrochlore rims.

Table 1. Chemical compositions of representative pyrochlore crystals from the titaniferous carbonatites and the adjacent pyrochlore-rich areas in dolomite carbonatites of the Petyayan-Vara.

Components U-Th (1) 1 Ca-F (1) Ca-F (2) Ca-F (2) Pb (1) Pb (1) Pb (2) Pb (2) BaO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. K2O b.d.l. b.d.l. 0.06 0.04 b.d.l. b.d.l. b.d.l. b.d.l. Na2O b.d.l. 4.66 6.84 6.73 0.36 b.d.l. b.d.l. b.d.l. SrO 4.09 1.41 1.19 1.25 1.42 1.80 2.00 3.13 CaO 5.88 15.48 16.49 16.25 4.02 1.05 1.17 1.51 PbO b.d.l. b.d.l. b.d.l. b.d.l. 32.14 31.10 26.81 25.41 UO2 3.18 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. ThO 9.71 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. Y2O3 b.d.l. 0.72 0.43 0.42 b.d.l. b.d.l. b.d.l. b.d.l. La2O3 0.42 b.d.l. 0.40 0.51 b.d.l. 2.20 2.92 2.55 Ce2O3 2.47 0.66 0.74 0.93 0.54 4.21 5.75 5.29 Nd2O3 0.44 b.d.l. 0.19 0.20 b.d.l. 0.33 0.51 0.21 Sm2O3 b.d.l. b.d.l. 0.20 0.22 b.d.l. b.d.l. b.d.l. b.d.l. SiO2 3.42 b.d.l. 0.12 0.26 0.47 0.16 0.26 0.60 TiO2 8.94 4.95 4.74 5.12 2.72 3.33 3.21 2.22 Fe2O3 3.24 0.39 0.11 0.17 0.71 0.57 0.73 0.61 ZrO2 b.d.l. b.d.l. 0.30 0.48 b.d.l. b.d.l. b.d.l. b.d.l. Nb2O5 41.74 57.49 64.39 64.60 42.00 44.09 46.11 48.73 Ta2O5 b.d.l. b.d.l. 0.17 0.10 b.d.l. b.d.l. b.d.l. b.d.l. F b.d.l. 3.80 2.84 2.85 b.d.l. b.d.l. b.d.l. b.d.l. Total 83.53 89.56 99.21 100.13 84.38 88.84 89.47 90.26 F=O 0.00 1.60 1.20 1.20 0.00 0.00 0.00 0.00 Total 83.53 87.96 98.01 98.93 84.38 88.84 89.47 90.26 Formula calculated to 2 B-site cations Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.00 0.60 0.80 0.77 0.06 0.00 0.00 0.00 Sr 0.15 0.05 0.04 0.04 0.07 0.09 0.10 0.15 Ca 0.40 1.10 1.07 1.03 0.39 0.10 0.10 0.13 Pb 0.00 0.00 0.00 0.00 0.78 0.73 0.60 0.55 U 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Th 0.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y 0.00 0.03 0.01 0.01 0.00 0.00 0.00 0.00 La 0.01 0.00 0.01 0.01 0.00 0.07 0.09 0.08 Ce 0.06 0.02 0.02 0.02 0.02 0.13 0.17 0.16 Nd 0.01 0.00 0.00 0.00 0.00 0.01 0.02 0.01 Sm 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Σ cat (A) 0.81 1.80 1.96 1.90 1.33 1.13 1.08 1.07 Vac (A) 1.19 0.20 0.04 0.10 0.67 0.87 0.92 0.93 Si 0.22 0.00 0.01 0.02 0.04 0.01 0.02 0.05 Ti 0.42 0.25 0.22 0.23 0.19 0.22 0.20 0.13 Fe 0.17 0.02 0.01 0.01 0.05 0.04 0.05 0.04 Zr 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 Nb 1.19 1.73 1.76 1.73 1.72 1.73 1.73 1.78 Ta 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Σ cat (B) 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 F 0.00 0.80 0.54 0.53 0.00 0.00 0.00 0.00 1 U-Th—U-Th-rich hydroxycalciopyrochlore, Ca-F—ﬂuorcalciopyrochlore, Pb—kenoplumbopyrochlore; (1)—crystals from the titaniferous carbonatites, (2)—from the pyrochlore-rich areas. b.d.l.: below detection limit.

5. Discussion The distribution of titanium oxides in all studied rocks was constrained by networks of cracks. Thus, titanium oxides were deposited at the postmagmatic stage, after the rocks were deformed and brittle. Several mixed-aged generations of these constraining cracks were observed in titaniferous carbonatites. The earliest cracks were dampened by imposed processes (K-feldspar formation, Geosciences 2018, 8, 281 13 of 19

apatitization, etc.), indicating the first portions of TiO2 to be deposited before the main stage of the metasomatic reconstruction. The general order of mineral paragenesis in titaniferous carbonatites is as follows: (1) anatase, (2) Nb-poor brookite, (3) rutile, and (4) Nb-rich brookite. This sequence is reduced up to the first two members in TiO2-carbonate veinlets from primary dolomite carbonatites in rimmings of Ti varieties. As mentioned, brookite-anatase glomers from TiO2-carbonate veinlets and brookite segregations of I type (associations of elongated crystals) from titaniferous carbonatites were definitely similar. Obviously, the appearance of dolomite carbonatites with TiO2-carbonate veinlets provides an idea about how “ordinary” carbonatites turned into titaniferous carbonatites at the earliest stages. Here, anatase must be more widespread, but the majority was paramorphically transformed into brookite after fluid processing. Simultaneously, the main amount of brookite from titaniferous carbonatites, which cemented earlier titanium oxides and other minerals, formed under the K-metasomatism. The difference in segregation forms and distribution scope of the “cementing” brookite produced a wide range of TiO2 morphotypes. Judging by a number of admixtures, rutile was altered during the apatitization imposed on the K-metasomatism. Considering the structural features, this mineral paramorphically substituted brookite. As noted above, despite a mosaic distribution of elements (niobium, first of all), amoeboid isolations of TiO2 included individual monocrystals, where brookites of pre-(Nb-poor) and postrutile (Nb-rich) generations were intergrown. Recrystallization appears to have occurred when the late (Nb-rich) generation of brookite formed after rutile. The stability field of rutile and the limiting P-T conditions for anatase-rutile and brookite-rutile conversions were reliably determined. However, the stability fields of anatase and brookite overlap each other ([40]; see their Figure1). Therefore, during the synthesis of titanium dioxide, obtaining a mixture of two metastable low-temperature phases (brookite and anatase) is common, whereas serious efforts are required to selectively obtain anatase or brookite (e.g., [41,42] and many others). The leading role in the formation of a particular polymorph in chemical synthesis (and, most likely, in natural systems) acquires the characteristics of the fluid, including chemical composition, Eh, pH, etc. In this case, a substitution of one metastable phase by another, especially considering the paramorphic transitions, resulted in a dynamic change in the composition of the fluid in the course of mineral formation. However, even a short-term increase in temperature should have led to an increase in the stability of the high-temperature modification and, therefore, to irreversible partial or complete replacement of metastable brookite and anatase with a stable rutile phase [43]. As such, the sequence and completeness of conversion into rutile largely depended on the composition of the precursor (brookite or anatase). For instance, niobium was found to inhibit the brookite-rutile reaction [44], and Hanaor and Sorrell [45] showed that the anatase-rutile transition is hampered not only by niobium, but also by many other elements. Thus, the change in TiO2 politypes is an evolutionary sequence in which members are closely related to the changing conditions of associated metasomatic reconstructions in rocks. In the current sequence, the niobium content reasonably increased, as detected in other similar objects as well [23]. Formed at the peak of niobium stocks in titaniferous carbonatites are (1) the above mentioned post-rutile Nb-rich generation of brookite in various morphotypes of TiO2-segregations, and (2) idiomorphic brookite (V type) and pyrochlore in association with dolomite from by-selvedge areas of cutting carbonatite veinlets. At this stage, niobium rims occur in dolomite carbonatites from edging of titaniferous varieties. These rims impregnated anatase and brookite that formed earlier. Areas with the pyrochlore mineralization occurred at that time. As stated above, many studies of carbonatite objects consider pyrochlore a sensitive indicator of processes imposed on a rock. Observing, though, the same changes in the pyrochlore chemical activity in different complexes, we tend to disagree on the nature of these phenomena. First of all, these phenomena concern the dividing of the late metasomatic and hypergene stages of the pyrochlore Geosciences 2018, 8, 281 14 of 19 genesis. Based on an analysis of the literary data, the evolution of the pyrochlore composition could follow this general order:

(1) Fluorcalciopyrochlore with the fully filled A-sites is the main variety in magmatic calcite [29,46–48] and dolomite [31,49] carbonatites. Th-U [50] varieties are less frequent, also with a minor amount of vacancies in A-site and dominant F in Y-site. (2) Fluorcalciopyrochlore is naturally substituted by varieties, whereas in the A-site (1) Ca and Na contents decrease, (2) cations become minor up to xenopyrochlores (A-vacancy > 1 a.p.f.u), and (3) Ba, Sr, REE, Th, U, Pb, and H2O become the dominant cations. In the B-site, a decreasing trend in Nb content isomorphically substituted by Ti and Fe occurs. Y-site is characterized by F decreasing until it disappears, and OH-groups occur in this position. Different authors have linked this chemical specific feature of pyrochlores with magmatic [51], hydrothemal [27,28,31,49,52–54], or supergene [55,56] processes. The “hydrothermal” point of view prevails. (3) In pyrochlores that are most reasonably referred to supergenous [17,49,56,57], the same chemical features are the most manifested. Ba, Sr, REE, Th, U, Pb, H2O, and rarely K remain the dominant cations in the A-site, but in general, the occupancy rate of this position drops to the lowest level, down to zero. Ca and/or Na tend to disappear. No fluorine-varieties are detected among supergenous pyrochlore. They differ from “hydrothermal” pyrochlores with the B-site completely filled with niobium. With a vacant A-site, it makes supergenous pyrochlores become the richest in niobium: 75 wt % vs. 50 wt % in other varieties.

U-Th and Ca-F-varieties represent early generations of the Petyayan-Vara pyrochlore and are commonly interpreted as magmatism products. However, the structural position of the pyrochlore localized in mineralized veinlets supports its fluid nature, which is also indicated by some of its chemical features. Thus, the earliest U-Th pyrochlore lacked A-site filling and fluorine. According to their characteristics, they are similar to pyrochlores from ankerite carbonatites of some Indian complexes [58–60]. The studied fluorcalciopyrochlore is compositionally identical to magmatic carbonatites. However, we note that fluorcalciopyrochlore from the well-known Nb-REE Bayan Obo deposit was originally described as a product of REE-, F-rich postmagmatic hydrothermal solutions impact on Ca-Mg-carbonatites [61]. Moreover, in exocontact rocks (fenites) of late carbonatites a priori fluid pyrochlore mineralization is also usually represented by fluorcalciopyrochlore [56,62]. We suggest both U-Th, and Ca-F-varieties of pyrochlore formed when apatite occurred in titaniferous carbonatites, which is indicated by the similar chemical features of both minerals (e.g., increased contents of Th, Y, Sr, etc.). The pseudomorph substitution of Ca-F-pyrochlore by a rare fluorine-free Pb-variety is a specific feature of the Petyayan-Vara carbonatites. Commonly, the transition passes through a number of intermediate areas of Ba-, Sr-, and REE-pyrochlores [28,63]. The pyrochlores reflect the general pattern of imposed metasomatic processes in such areas, including the Petyayan-Vara ore field rocks. Direct transitions to Pb-pyrochlore are only present in fenite around the Chilwa Island Carbonatite, Malawi [64]. An association of Pb-pyroclore and later monazite is typical in both cases, as well as the mentioned fenites. We presume the absence of transition areas to be a result of specific geological features of the Petyayan-Vara titaniferous carbonatites. These rocks are supposed to have almost no imposed Ba-, Sr-, or REE-metasomatic processes due to their dense fine-grained structure obtained during recrystallization. We think that the peculiar evolution of pyrochlore in titaniferous carbonatites of the Petyayan-Vara confirms the conception that emerged in our minds during this field study that was strengthened while analyzing the geochemistry of the rocks. Apparently, within the Petyayan-Vara carbonatite field (1) there are products of all major processes typical of late carbonatites and (2) these products are often spatially isolated. Therefore, the Petyaya-Vara rocks provide a unique ground for the differentiated study of the late carbonatite genesis. Geosciences 2018, 8, 281 15 of 19

Despite this, the formation mechanism of titaniferous carbonatites is still not completely clear. Large-scale metasomatic transfer of titanium is assumed to be hardly possible [65]. Therefore, the probable source of titanium and niobium are the enclosing aluminosilicate rocks. Remobilization of components from the latter by carbonatite-hosted fluids can also explain other specific geochemical features of titaniferous carbonatites. For example, their enrichment in such atypical elements for carbonatitic melts as Al and Si. However, we cannot unequivocally deny the probability of HFSE introduction by fluid from a remote source. As mentioned before, the rare-earth carbonatites of the Petyayan-Vara are predominantly ancylitic. This is rather unusual, since in most such complexes, the main REE-carbonate is bastnaesite [66]. However, this is not unique. The second largest deposit of rare earths in the U.S. (Bear Lodge alkaline complex, Wyoming) is also ancylitic [67]. Based on the available information on the geology of the Bear Lodge REE carbonatites [13], the similarity of the complexes is both mineralogical and geochemical. Within the Bear Lodge complex, zones with HFSE-mineralization occur but they are larger than those at the Petyayan-Vara. The rocks of one of such zones, known as the Cole HFSE(+HREE) Occurrence, contains up to 44.9% TiO2, 3.1% Nb2O5, 6.5% Y2O3, 0.8% Dy2O3, 2.6% ThO2, 6.0% P2O5, and 3.7% F. They are composed of K-feldspar (30–40%) and Nb-rich anatase (30–50%) [68]. In contrast to the Petyayan-Vara, these rocks are located outside the massif (about two km from the main carbonatite intrusions) among sedimentary rocks poor in the listed components. Such geological conditions allowed Andersen et al. [ibid.] to conclude that “ ... HFSEs and HREEs are transported in highly fractionated F-rich fluids of high-ionic strength that originated from carbonatite intrusions or associated carbohydrothermal residua”. Considering the similarity between the massifs in general and the HFSE-rich rocks in particular, we admit the possibility of Ti-Nb fluid influx from deeper levels of the Petyayan-Vara complex. Regardless of the source, titanium and niobium apparently migrated together. Complexes with 2− 3− halogens ([69] and its references) and, to a lesser extent, CO3 and PO4 ligands [70], played a decisive role in the transfer of these elements. In our case, the main transfer agent was fluorine. This is indicated by the presence of fluorapatite, fluorcalciopyrochlore and F-rich phlogopite in the rocks. The intergrowths of Ti-oxides with sulphides may indicate the involvement of S-bearing ligands. As with the formation of HFSE-rocks in the Bear Lodge, the deposition of Ti in the Petyayan-Vara’s titaniferous carbonatites was probably facilitated by a decrease in temperature and fluid-carbonatite interactions. However, unlike most similar massifs, fluorite was not formed. This preserved niobium in the fluid phase. Moreover, according to recent studies [71], when the high HF content in solution is high, a decrease in temperature could lead to an increase in the solubility of Nb as niobium hydroxyl-fluoride species and its accumulation in the residual fluid. Further migration of this residual fluid to adjacent dolomite carbonatites formed the chain-like pyrochlore mineralization and Nb-rich rims in anatase and brookite from TiO2-carbonate veinlets. The apatitization process could trigger niobium deposition (the formation of the late Nb-rich generation of brookite and fluorcalciopyrochlore) in the titaniferous carbonatites. Apparently, this process caused the deposition of fluorine from the fluid. Simultaneously, temperature increased, which caused a paramorphic conversion of brookite into rutile.

6. Conclusions and Perspectives The peak of niobium stocks in the Vuoriyarvi massif was during the magmatic stage of its genesis, as found in other carbonatite massifs around the world. It produced the Neske-Vara pyrochlore deposit. However, titanium concentrated under the formation of late (titaniferous) carbonatites before the Ba-Sr-REE metasomatism. Fluids played the main role in the genesis of the studied rocks. Judging by dynamically swapping titanium oxides and widespread paramorphic transitions, this pre-REE stage was associated with actively changing physical and chemical parameters. Niobium accumulated in specific residual fluids, low in titanium at that time. The impact of this residual fluid led to the formation of pyrochlore, late generation of Nb-rich brookite in titaniferous carbonatites and Nb-rich rims impregnating early generations of anatase and brookite Geosciences 2018, 8, 281 16 of 19

from TiO2-carbonate veinlets. This created the second peak in niobium stocks, but not as large-scale, as the one occurred during the magmatism. Along with Mg and Fe, titanium and niobium were the most mobile components in the considered metasomatic processes. Even the inflow of mobile elements, such as alkali, was a catch-up, as indicated by pyrochlore resorbed by phlogopite. Thus, we obtained a general view at the chemical separation of HFSE from other elements and their isolation. The studied processes occurred at a specific stage of the matter differentiation, which preceded the formation of rare-earth carbonatites and is likely to be tightly interwoven. Since almost no publications on late carbonatites (including REE) goes without mentioning Nb-rich titanium oxides, we consider it important to address the following issues that are both practically, and theoretically topical: the source of Ti-Nb fluids and the mechanism of their formation. Unfortunately, the data required to answer these questions are still lacking. We expect that the answer will be determined through studies of fluid inclusions, as well as radiogenic and stable isotope systematics.

Author Contributions: E.K. conceived and designed the experiments; E.K., E.F. and M.S. conducted field research; E.K. studied petrography and mineralogy of rocks; E.F. described the mineralogy of TiO2 phases; M.S. investigated the pyrochlore mineralization; V.S. made electron back scattered diffraction (EBSD) measurements, backscattered electrons (BSE) images and analyzed the chemical composition of the minerals. All the authors participated in the writing of the paper. Funding: This research was funded by the Russian Foundation for Basic Research grant number 16-35-00132]. Acknowledgments: This work was carried out in the Geological Institute KSC RAS under the state order No. 0231-2015-0009. The authors express their sincere appreciation to T.A. Miroshnichenko for help with translation. We also thank A.V. Chernyavskiy (GI KSC RAS) for his help in the field works, as well as V.N. Bocharov (RC “Geomodel” SPbSU) and A.V. Bazai (GI KSC RAS) for their assistance in studying the structure and composition of the minerals. The authors express their sincere gratitude to two anonymous reviewers. Conflicts of Interest: The authors declare no conflict of interest.

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